# B Dynamic Fluid (airplane landing/decrease altitude)

1. Feb 21, 2017

### Arif Setiawan

Hi guys.. I've some discussion with my friend. We are teacher. So we must carefully to answer question for student.. there the question is

From figure below, p2 and p1 applied on plane's wings. When airplane landing/decrease altitude, wich condition must occupied
A. P1=P2 and v1=v2
B. P1<P2 and v1>v2
C. P1<P2 and v1<v2
D. P1>P2 and v1>v2
E. P1>P2 and v1<v2

On my opinion, at any rate we total force by bernoully equation must upright vertical oposite weight. Because, without Force of wings plane will fall. So, when take off/increase altitude F>w .. stay at certain height F=w.. and when landing F<w.. But F must remain positive. So P1>P2 and v1<v2 (E)

At opposite, my friend though that when take off P1>P2 and when landing P1<P2.

2. Feb 21, 2017

### Staff: Mentor

You are correct: when decreasing in altitude, the wing is still producing positive lift...just less of it.

3. Feb 21, 2017

### rcgldr

In a steady descent, the total net force is zero (otherwise the aircraft would accelerate). A component of drag in addition to most of the lift opposes gravity. In a steady climb, lift is also less than the weight, and a component of thrust (in excess of drag) opposes gravity.

Last edited: Feb 21, 2017
4. Feb 21, 2017

### Staff: Mentor

Thanks for the clarification. I did fall into a little trap there, though it doesn't impact the problem answer.

5. Feb 21, 2017

### rcgldr

If the aircraft is not accelerating, then total net force = zero. In level flight, lift = weight (ignoring direction). In a steady climb or steady descent, lift < weight (ignoring direction). In a steady climb, a component of thrust (minus drag) opposes gravity. In a steady descent, a component of drag (minus thrust) opposes gravity.

6. Feb 21, 2017

### Arif Setiawan

That's the aspect that I havent see before..
Thanks for these clear information